This invention relates to the manufacturing apparatus and method of the preform that is the raw material rod of the optical fiber used in the fields such as telecommunication.
The optical fiber is manufactured in the following procedures. First, the soot of the quartz as synthesized by the VAD (Vapor Axial Deposition) method is dehydrated and sintered into an ingot of synthetic quartz. This ingot is then heated in a furnace, drawn under heating and reduced in diameter into a rod, namely the preform for optical fiber.
An ingot drawing apparatus is disclosed in the Japanese Patent Provisional Publication No. 7-2539. The ingot is fed from the top of a heating furnace. The rod as drawn under heating condition is taken out from the bottom of the furnace. The diameter of glass rod coming out of the furnace after drawing is measured at a position. Based on the temporal variation in the measured values, the rod diameter after drawing is controlled in compliance with a process model that can compensate for the idle time.
Because the preform for optical fiber that the authors of this invention are going to manufacture uses, as raw materials, far greater diameters of synthetic quartz ingot and the rod after drawing, it is necessary for us to have a larger size of the heating furnace. Such large size of furnace tends to disturb the air current in it and/or the temperature distribution due to the displacement of the ingot. These disturbances having a large influence over the rod diameter of the preform for optical fiber thus manufactured, an attempt to control the drawing rate based on the outer diameter of the rod coming out of the furnace will cause some hunting. This varies the rod diameter cyclically, which brings obstacle to the manufacture of a preform for optical fiber with stable diameter. Further, the disturbance in temperature distribution may sometimes curve the rod or deform its outline into elliptical.